The present invention relates to an electric energy meter, and more particularly to a method and apparatus for detecting and compensating for reverse rotation and creep of the energy meter eddy current disk.
Induction watthour meters conventionally utilize the rotation of an eddy current disk in response to the rate of power consumption by the load on the lines being metered, which is then totalized or integrated as a measure of power consumption, usually expressed in kilowatt hours. Such induction watthour meters commonly include electronic registers for providing time of use or demand energy information which is also used for billing purposes. In such electronic registers, a series of electrical pulses is used to provide an electrical signal responsive to the rate of energy consumption. This is usually achieved by mounting a castellated or toothed optical shutter on the eddy current disk shaft of the energy meter for rotation with the shaft, and positioning a light source and a light detector on opposite sides of the shutter. The toothed shutter interrupts the light path between the light source and the light detector as it rotates between them and provides pulses at a rate proportional to the rate of energy consumption by the load on the power lines being metered. U.S. Pat. No. 4,665,358, issued May 12, 1987, to D. F. Bullock, F. Y. Simon and R. G. Farnsworth, assigned to the same assignee as the present invention, and hereby incorporated by reference, discloses one electro-optical method of determining the direction of rotation in an electric energy meter based on frequency and/or phase differences, and comparison of a new and a preceding state. The present invention constitutes an improvement, and extension of the use, over that disclosed in the aforesaid U.S. Pat. No. 4,665,358.
There are typically one or two anti-creep holes provided in a single-phase energy meter eddy current disk to lock the disk in a fixed rotational position in the absence of energy comsumption by the load being metered until dislodged by a substantial torque upon the resumption of energy consumption by the load being metered. In a properly adjusted energy meter, the two anti-creep holes prevent the eddy current disk from turning more than 180 degrees when power is not being consumed, and also prevents disk "pull-back" of more than 180 degrees after current has been removed. However, a meter can be improperly adjusted such that one of the anti-creep holes is not effective. This would allow the disk to move almost 360 degrees when power is not being consumed or when the load current has been removed. Under such conditions, it is possible that the rotational pulses from the electronic register may be counted twice, once when the eddy current disk pulls back or creeps in the reverse direction, and again when the disk moves forward through the portion of the reverse rotation back to the original position where the reversal began. Since the number of pulses counted is used in determining customer billing, it is very important that energy meters used for billing purposes be highly accurate and avoid billing errors. In addition, it is desirable not only to compensate for errors due to "pull-back" and creep of the eddy current disk, but to accomplish this without adding additional components or increasing the manufacturing time or expense.
In addition, persons tampering with an energy meter have been known to reverse the connections of the energy meter and to employ other methods designed to cause the eddy current disk of the energy meter to rotate in the direction which is the reverse of normal rotation. Such a reversal for a portion of a billing period can cause a subtraction from the meter reading resulting in a lower, but false, indication of the amount of energy consumed, and leading to a lower bill than appropriate for the actual energy consumed. On the other hand, there are installations where reverse flow of current in an energy meter can be encountered in normal operation. Accordingly, it becomes important to detect and meter the reversal of rotation of an energy meter for an extended amount, that is an amount in excess of the smaller amount of reverse rotation resulting from pull-back, chatter, or creep of the eddy current disk. In addition, correction of the readings to eliminate the effects of pull-back and creep is also desirable.